1. Field of the Invention
The present invention disclosed and claimed herein relates to an improved piston used in the engine of a motor vehicle.
2. Description of the Prior Art
Japanese Patent Publication No. 59-25868 shows a piston, which on its top land has aluminium powder, 2-sulfer-molybdenum, epoxy resin, and other component mixed in a certain ratio, and applied in a thickness greater than 30 .mu.m. The form of the piston is shaped to correspond to the form of the cylinder as the engine operates. Consequently, the hydrocarbon content of the combustion gas is reduced, and a rise in temperature of the ring-groove is prevented.
However, the above-mentioned materials are very expensive, and furthermore the application of these materials to pistons is expensive because a piston must be coated many times with the materials to obtain the desired thickness. Thus, the working line required to apply these materials to a piston is excessively long. In addition, the epoxy-resin contemplated for use sometimes breaks off and interferes with the cylinder face.
Further, the thermal expansion coefficient of aluminum, which is a main material of the piston, is about double that of cast steel which is a material of the cylinder block. Thus, the top land of the piston expands more than the piston ring land part as a result of the combustion temperature. Therefore, the clearance between the cylindrical surface of the top land and that of the cylinder is normally designed to be from 0.4 mm to 0.6 mm, so that the top land does not slide heavily on the inner wall of the cylinder during engine operation. But, in order to reduce the hydrocarbon (HC) content of the exhaust gas, a reduction in the clearance provided between the surfaces of the piston and the cylinder is needed, and in providing the same the eccentricity of the cylinder hole, the profile between the skirt shoulder and the skirt end, and the overall profile of the piston must be considered. Also, in the assembly of the cylinder head, transmission, exhaust manifold and engine manifold to the cylinder block thermal deformation of the respective cylinder bores due to the unequal cooling of each must be taken into account. Therefore, although the most suitable diameter of the top land of the respective pistons may be different, in practice it is not practical to permit this so that a compromised, suitable, uniform top land diameter must be used.